Removing the toxic pollutants in automotive exhaust has been an intense focus of the automotive industry over the last several decades. In particular, the emissions regulations for fuel-efficient diesel engines that were implemented in 2007 and 2010 have resulted in a new generation of emissions control technologies. One of these technologies that has a multifunctional purpose is the diesel oxidation catalyst (DOC). Its function is primarily to oxidize carbon monoxide (CO) and unburned hydrocarbons (HCs) to carbon dioxide (CO2) and water (H2O), but also is used to oxidize a portion of the nitric oxide (NO) to nitrite (NO2) for use in low temperature soot oxidation in the diesel particulate filter (DPF) and to enable the optimal NO/NO2 ratio for the selective catalytic reduction (SCR) of NOx with NH3 over metal-zeolites.
The active component of the DOC is typically Pt and/or Pd. These catalysts usually reach 90% conversion between 200° C. and 350° C., and consequently, the catalysts are not active under “cold-start” or low load/idling conditions. The temperature limitation is problematic since more than 50% of the emissions from an engine occur in the first 2-3 minutes after cold start. Thus, as emissions regulations become more stringent, meeting the emission regulations will require increased activity during this warm-up period. To further complicate matters, the increased Corporate Average Fuel Economy (CAFE) standards that will be implemented over the next decade will result in the introduction of more fuel-efficient engines. These engines will have even lower exhaust temperatures, which further necessitates the need for increased emissions control activity at low temperatures. Higher Pt/Pd loadings may help to increase the catalytic efficiency, but such methods would be too expensive and are more subject to particle sintering which degrades performance over time.
Other options to meet the emissions standards include hydrocarbon/NOx absorbers which operate by trapping pollutants below temperatures at which catalysts would treat them and, subsequently, releasing the pollutants once temperatures rise to a point at which a downstream catalyst can effectively treat them. However, while this pathway would help mitigate emissions at low exhaust temperatures, additional complexity and cost are drawbacks to implementation.
Thus, oxidation catalysts in emissions control systems, such as automotive exhaust that generally contain Pd/Pt require exhaust temperatures above 200° C. to operate; and under low-temperature conditions, oxidation of CO and hydrocarbons are challenging. As engine efficiency improves and exhaust temperature decreases, there is an increasing demand for high emissions control performance at low temperatures. Therefore, it becomes imperative to design new catalysts that are active at low operating temperatures.